Analog-to-digital code conversion has in the past been accomplished in a number of different ways, commencing, long ago by use of various discrete-component hardware implementations and proceeding through periodic generations generally trending toward the development and production of integrated circuit devices intended for universal or widely-applicable use as "building block" converters, which have come to be used on a more or less generalized basis, in a multitude of applications, wherever the need presented itself.
The use of such "off the shelf", multi-purpose A/D converters has led to almost immediate acceptance and widespread use which to a considerable extent has replaced particularly-designed hardware converter implementations; however, it is quite likely that in a number of such instances individually-designed converter systems would yield both improved results and economic advantage as well, and the present invention is addressed to situations of this type.
In a particular illustrative embodiment, the present invention is directed to "embedded servo" systems such as (by way of particular example) that disclosed in previously-filed applications for U. S. Patent Ser. No. 280,138, now U.S. Pat. No. 4,472,750, issued Sept. 18, 1984, Ser. No. 589,007, now U.S. Pat. No. 4,586,094, issued Apr. 29, 1986, which are assigned to the same assignee as the present invention. In the systems particularly addressed in these prior applications, a format for embedded servo information is disclosed which includes positioning information blocks containing both track-identification information and servo-positioning tracking "bursts". In a preferred form, the track-identification information comprises a sequential series of timed bursts of constant-amplitude and constant-frequency signals which are formatted to occur in "bit cells", the arrangement being such that a sequence of such bursts comprises a series of individual bit cells, and in each such bit cell the relative duration of the burst is indicative of a binary zero or one in digital value. In this manner, the numerical designation of each recording track on the media may readily be encoded in binary form for subsequent detection and decoding by the transducer head when it is in a position of alignment with respect to that particular track. Further, disposed in a given time relationship with respect to such track-identification bursts, paired servo-tracking signals are recorded, for example by first and second ("A" and "B") bursts which are positioned on opposite sides of the track centerline and which appear in predetermined timed sequence with respect to one another. Such tracking bursts are detected and decoded in an amplitude-representative manner, i.e., with opposite but equal amplitudes in the event the head is precisely centered on the track centerline, and with progressively different relative amplitudes resulting from tracking displacement of the head to one side or the other of the centerline, differencing of the "A" and "B" signal values producing an error signal which can be used in closed-loop servo-controlled tracking procedures.
In an environment such as the one just described, digital processing is an integral part of the overall operation of the system, whose purpose is for the storage and retrieval of user data, typically recorded with very high-density bit streams in a modified FM recording format. It is thus desirable and perhaps essential to use such digital processing techniques (e.g., by microprocessor control) in decoding and utilizing the positioning information, but it does not necessarily follow that the most effective and advantageous systems to be employed for this purpose should utilize "off the shelf" integrated circuit A/D converters, either for the quantification of the A and B servo-positioning bursts, or for the decoding of the track-identification information.